1. Field of the Invention
This invention relates to a gear made of plastic (hereinafter referred to as a plastic gear), which includes a ring-like hub forming a core portion, a concentric gear ring which has a larger diameter than that of the hub, on an outer surface of which gear teeth are arranged, and a plate-like web which connects the hub with the gear ring in a body. In particular, relates to a plastic gear having high productivity and capable of avoiding a vibration in a transmission device.
2. Discussion of the Background Art
It is well known in a transmission apparatus to have at least one plastic gear which transmits a rotation force of a driving motor to a driven member. For example, in an image forming apparatus such as a copier, a printer, a facsimile or a multi-functioned machine having a plurality of functions, a rotation force of a driving motor is generally transmitted to an image carrier through a driven member contacting the surface of the image carrier or the like for forming a toner image on the surface of the image carrier during its rotation. The conventional plastic gear may also be used in a duplicator, a camera, a video deck or a compact disk player and so on to transmit a rotational force to a driven member thereof.
In recent years, such plastic gears have tended to be rotated at higher speeds and so have been subjected to higher external forces. Since the conventional plastic gear is simply constituted by a hub, a gear ring and a web connecting the hub with the gear ring, it has been difficult to meet the necessary level of rigidity and strength required for the plastic gear. It is of course possible to increase both the rigidity and the strength to meet the prescribed level if both a thickness and a size of the gear are increased. However, this is costly and the transmission apparatus unavoidably becomes bulky.
To increase the rigidity, a plurality of ribs may be symmetrically mounted on both front and rear surfaces of the web in a body in a manner such that one edge connects to the hub and another edge connects to the gear ring. However, the diameter of such a plastic gear generally varies during its molding process due to the so called a shrink phenomenon of the plastic. As a result, a peripheral speed of the plastic gear periodically changes when it rotates, and accordingly, unevenness of the rotational speed of the driven member may arise.
The present inventor has determined that the shrink phenomenon occurs for the reason hereinbelow explained in detail referring to FIGS. 10 through 12. A conventional plastic gear 14 includes a cylindrical hub 25 disposed as a core portion and supported by the shaft 15 illustrated in FIG. 2. The plastic gear 14 further includes a gear ring 27 which is substantially concentric with the hub 25, has a larger diameter than the hub 25, and is disposed outside of the hub 25.
The gear ring 27 includes a plurality of gear teeth 26 on an outer circumferential surface thereof. The plastic gear 14 further includes a circular plate-like web 28 which connects the hub 24 with the gear ring 27 to form a single body. A plurality of ribs 29A and 29B are integrally formed respectively on a front side and a rear side of the web 28. Each of the ribs extends radially from the hub 25 to the gear ring 27.
The ribs 29A formed on a front surface of the web 28 are arranged at a prescribed angular interval around the hub 25. The ribs 29B formed on a rear surface of the web 28 are arranged in a same way as the ribs 29A. The ribs 29A and 29B are disposed at the same angular positions on the front and rear surfaces of the web 28. As a result, a perpendicular cross section of the web 28 intersects the cross sections of both the ribs 29A and 29B, as illustrated in FIG. 7. Since a partial shrink phenomenon occurs at each of portions of the plastic gear 14 where the ribs 28A and 29B are symmetrically formed on the front and rear side surfaces of the web 28 during a cooling process of molding, diameters of these portions decrease to be less than that of other portions.
When producing a gear made of a metal by cutting a metal material, such a partial shrink phenomenon, of course, does not occur. Such a partial shrink phenomenon may occur only in a case that a pair of ribs 28A and 29B are symmetrically formed on the front and rear side surfaces of the web 28. The eccentricity of a gear periphery of the conventional plastic gear that includes six pairs of ribs 29A and 29B respectively formed on the front and rear side surfaces 28A and 28B of the web 28 is illustrated in FIG. 14. As there seen, the gear diameter changes six times corresponding to the number of the ribs. As a result, the rotational speed of the plastic gear varies six times; thereby unevenness of the rotational speed arises when the conventional plastic gear rotates.
A possible cause of the change in rotational speed of the plastic gear is explained below. A portion of the gear ring 27 and gear teeth 26A, 26B and 26C each mounted on the circumference of the gear ring 27 are typically illustrated in FIG. 15. As there shown, ends of the ribs 29A and 29B are connected to the same portion (shown enlarged for ease of illustration) of the gear ring 27 between the teeth 26B and 26C. The portion of the gear ring 27 between the teeth 26B and 26C is more indented toward a rotational center of the plastic gear than other portions thereof, since the partial shrink occurs when the plastic gear is molded. Thus, the tooth 29A positioning at a left side of the ribs 29A and 29B inclines to the right and the tooth 29B positioning at a right side of the ribs 29A and 29B inclines to the left as illustrated in FIG. 15.
A gear tooth 26D of another gear meshes with the plastic gear 14 as illustrated in FIG. 15. If a pressure angle at a gear connecting portion at which the gear tooth 26D meshes with the gear tooth 26A is .alpha..sub.0, a pressure angle .alpha..sub.1 of the gear tooth 26B inclining on the right is larger then .alpha..sub.0. A pressure angle .alpha..sub.2 of the gear tooth 26C inclining on the left is smaller than .alpha..sub.0.
If angular velocities are .omega..sub.0, .omega..sub.1 and .omega..sub.2 correspond to gear portions having the angles of .alpha..sub.0, .alpha..sub.1, and .alpha..sub.2, then the larger the pressure angle, the smaller the angular velocity and the smaller the pressure angle, the larger the angular velocity. Thus, the following relation is established around the ribs 29A and 29B: EQU .omega..sub.1 &lt;.omega..sub.0 &lt;.omega..sub.2
Thus, when ribs 29A and 29B extend in a radial state, for example, from the rotational center of the gear and are each disposed in the same angular interval, a rotational speed of the gear periodically varies when the plastic gear rotates.
Further, a rotational speed of the conventional driving motor 10 generally varies once per one revolution thereof. Thus, a rotational velocity of the PC drum 1 remarkably changes at a prescribed timing, if a frequency of a change in rotational speed of the conventional driving motor 10 is almost coincident with that of the plastic gear 14. This is because cyclical peaks due to the change in rotational speed of the driving motor 10 and that due to the plastic gear 14 coincide with each other. As a result, unevenness of a toner image (so called "jitter") arises on the surface of the PC drum 1, and the image quality is inferior.
For example, if the driving motor 10 rotates at 1,800 rpm, a frequency of a change in rotational speed is 30 Hz (obtained by dividing 1,800 rpm by 60 second). If the number of teeth of the output gear 13 of the driving motor 10 is ten, a number of teeth of a plastic gear 14 that meshes with the output gear is seventy, and a number of ribs 28A and 28B mounted on each of the surfaces of the web 28 of the plastic gear 14 is seven, a frequency of a change in rotational speed of the gear 14 becomes 30 Hz, as obtained by the following formula: EQU 1800 rpm.times.(10/70).times.(1/60 sec).times.7=30 Hz
Thus, if the peaks of the above-mentioned cycles accord with each other, the change in rotational speed of the PC drum 1 becomes remarkably large, since the change in rotational speed of the driving motor is added to that of the plastic gear 14. Thus, the above-mentioned plastic gear may not be used for the transmission device.
Further, a plastic gear may be produced using an injection molding method. In such a method, if a new plastic gear that has a larger or smaller number of ribs than the plastic gear previously used is to be molded using the same mold, the mold is required to be remodeled to produce a different number of the ribs. However, it generally takes a huge cost, if remodeling the mold that has produced the previous model of the plastic gear, for example by changing a plastic injection gate through which molten plastic is poured.